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Celestron C8 SCT with HEQ5Pro


biondi
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Hi,

I recently posted a similar message in the beginners forum asking whether a C8 SCT (on a HEQ5 Pro) for DSO imaging was just too high a focal length. I can't seem to delete that one at the moment though.

Having been looking in this section for a while and seeing some of the great images that you've been producing I think here would be a good place to ask however.

Does anyone image using a C8 here? I'd love to hear what guide scope you use (I have a Celestron 80mm) and how you've managed it.

cheers,

Matt

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Ths is not a setup I would consider for one second. It is not that it would be out of the question to try it, but unless you already have this setup why would you opt for it? In imaging the biggest pain is being under mounted. Now many beginners look at the weight of the OTA and the payload of the mount (halved for imaging, shall we say) and think, Yes, that would be OK. But that is the wrong way to think about it. You have to factor in the weight, the focal length, the focal ratio, the size of the pixels and the sensitivity of the camera to have an idea of how it is likely to behave.

Long FLs need better guiding.

Slow F ratios need longer subs (which need better guiding and lost long subs hurt more than lost short ones.)

Small pixels need both longer subs and better guiding. (Rule of thumb but respect it.)

Insensitive cameras need longer subs.

If you pick the 'wrong' options here you can end up with a double, triple or quadruple whammy working against you. My honest opinion? You won't win.

For all its virtues the C8 is a very ordinary bit optics for DS imaging, even on a premium mount. A much smaller apo will out resolve it. (The Edge is another matter but that has other issues and I don't think you're asking about that?)

If this is a blank piece of paper, don't do it. That would be my feeling. If you already have some or all of the kit then the question is, how much do you risk spending given that it might not deliver?

Olly

http://ollypenrice.s...39556&k=FGgG233

Edited by ollypenrice
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Thanks Olly, that's the kind of stuff I actually really want to hear. I already have this kit and when I approached a well known astronomy shop with what I wanted to do about a year ago (DSO imaging) this is what they sold to me. I didn't know any better, I know a little more now.

Perhaps I could get away with selling the C8 OTA and replacing it with something more fit for purpose. The HEQ5 Pro mount is fine I think but just needs something else on it.

Would you have any recommendations?

Many thanks,

Matt

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The HEQ5 is great. It would be a dream with a nice small refractor. If you are up the additinal complexities of Newt imaging then fine. I'd go for a small refractor. 10 million ED80s can't be wrong!

You were very badly advised regarding DSO imaging but the shops love unloading SCTs. They really do have their place but are over-sold mightily. Fellow victim!

Olly

Edited by ollypenrice
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Thanks guys. The ED-80, with an aperture of only around 3inches seems quite small and at f7.5 it doesn't seem particular quick. For DSO wouldn't you want something with a bigger aperture to get more light in? Or is the ED-80 really the right weight for AP with the HEQ5 Pro? I'm not saying money isn't an issue here but I'd happily wait and save for the right scope than make the same mistake as the SCT :) In your opinion, what's the best I could get OTA wise for this mount for DSO AP use?

Thanks again,

Matt

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I succesfully used a Meade LX200 8" on a HEQ5pro for a few years for spectroscopy...the C8 is even lighter....

I don't think you have a problems with carrying capacity...

As others say imaging at long focal lengths can be a PITA but I found it (spectroscopy - holding a star image on a 25 micron slit) doable with an beamsplitter at f10....

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Thanks guys. The ED-80, with an aperture of only around 3inches seems quite small and at f7.5 it doesn't seem particular quick. For DSO wouldn't you want something with a bigger aperture to get more light in? Or is the ED-80 really the right weight for AP with the HEQ5 Pro? I'm not saying money isn't an issue here but I'd happily wait and save for the right scope than make the same mistake as the SCT :) In your opinion, what's the best I could get OTA wise for this mount for DSO AP use?

Thanks again,

Matt

'Letting more light in' is a useful concept in visual observing. It has no meaning in AP. All scopes of aperture X let in the same amount of light but how much they concentrate onto a given pixel is determined by focal ratio. This may seem counter intuitive but a fast F ratio (low F number) gives a bright image quickly. Double the F ratio and your exposure time goes up 4x. I don't feel there's much wrong with small scopes. This was with 85mm. The fast F ratio (F3.9) and long exposure time have allowed it to go fairly deep. To catch the brown dust at F10 would take a lifetime!

Be aware that fast reflecting astrographs, even expensive ones, might best regarded as kits. They rarely perform from the box.

M45%20COMPOSITE%20FL-M.jpg

Olly

Edited by ollypenrice
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